This invention relates to the integration of microelectromechanical systems (MEMS) devices, such as sensors and actuators, and more particularly to an integrated MEMS system including integrated circuits (IC) and MEMS chips.
Micro-electro-mechanical system (MEMS) devices, in particular inertial sensors such as accelerometers and angular rate sensors or gyroscopes, are being used in a steadily growing number of applications. Due to the significant increase in consumer electronics applications for MEMS sensors such as optical image stabilization (OIS) for cameras embedded in smart phones and tablet PCs, virtual reality systems and wearable electronics, there has been a growing interest in utilizing such technology for more advanced applications which have been traditionally catered to by much larger, more expensive and higher grade non-MEMS sensors. Such applications include single- and multiple-axis devices for industrial applications, inertial measurement units (IMUs) for navigation systems and attitude heading reference systems (AHRS), control systems for unmanned air, ground and sea vehicles and for personal indoor GPS-denied navigation. These applications also may include healthcare/medical and sports performance monitoring and advanced motion capture systems for next generation virtual reality. These advanced applications often require lower bias stability and higher sensitivity specifications well beyond the capability of existing consumer-grade MEMS inertial sensors on the market. In order to expand these markets and to create new ones, it is desirable and necessary that higher performance specifications be developed. It is also necessary to produce a low cost and small size sensor and/or MEMS inertial sensor-enabled system(s).
Given that MEMS inertial sensors such as accelerometers and gyroscopes are typically much smaller than traditional mechanical gyroscopes, they tend to be subject to higher mechanical noise and drift. Also, since position and attitude are calculated by integrating the acceleration and angular rate data, respectively, noise and drift lead to growing errors. Consequently, for applications requiring high accuracy, such as navigation, it is generally desirable to augment the six-degree-of-freedom (6DOF) inertial capability of MEMS motion sensors (i.e., three axes of acceleration and three axes of angular rotation) with other position- and/or orientation-dependent measurements. Such sensor fusion is necessary for better results.
The success of the semiconductor industry has been driven by the ever-increasing density of devices on silicon integrated circuits. Historically, this increase in density has been accomplished by the reduction of electronic device feature sizes, through improved methods of photolithography and etching. Integrated circuit (IC) feature sizes have reached submicron dimensions with 180 nm being common, and 10-20 nm as the state of the art. These dimensions are approaching the limits of conventional semiconductor processes, particularly photolithographic optical limits. At the same time, overall chip functionality has been increased by expanding the lateral chip dimensions in 2D to create ever larger chips.
In practice, the electrical traces on the package substrates are much larger than those on the silicon chips. This mismatch leads to routing difficulties and excessive power consumption, and has led to the introduction of silicon (Si) interposers upon which fine signal distribution traces can be patterned on the chip side and coarse connections patterned on the substrate side with electrical interconnections between the two through the interposer. This approach using a Si interposer is referred to as 2.5D, since the chips are distributed in 2D, but the interposer has been introduced in the third dimension albeit with little additional functionality other than passive elements such as resistors and capacitors.
The development of through-silicon vias (TSVs) has enabled 3D integrated circuits (3DIC) in which the individual IC chips are thinned and stacked. The introduction of TSV technology to the IC process adds an additional level of complexity. IC processes are typically constrained to a few microns near the surface of the IC chip and require fine feature photolithography while the TSV processes are coarser features and penetrate the thickness of the IC. Thus, regions of the IC chip must be segregated for TSV fabrication, resulting in inefficient use of the silicon area and higher IC unit cost. Furthermore, the TSVs are typically metal-filled, particularly with copper. The copper cannot be part of the front-end process because of the temperatures needed to fabricate the IC circuits. Thus TSVs must be made of polysilicon if they are to be fabricated early in the process, or they must be fabricated at the end of the process if they are to be made of metal. Either approach adds complexity to a semiconductor process that is typically highly controlled and not amenable to modification.
In parallel with the efforts to integrate more and more electrical function into 3DICs, there is a desire to integrate MEMS into electronics. MEMS are integrated circuits containing tiny mechanical, optical, magnetic, electrical, chemical, biological, or other, transducers or actuators. As electronic devices include more and more features, designers need to include MEMS sensors to provide feedback for the user. For example, smart phones are incorporating MEMS accelerometers and gyroscopes to provide motion information for phone position, gesture-based instructions, navigation and games. As electronics become smaller, more complex, and more integrated, it is desirable to include MEMS devices in system chips, which include integrated circuits to process the MEMS signals. However, there are some fundamental differences between many MEMS fabrication processes and IC fabrication processes. For most MEMS devices, the MEMS mechanical element (e.g. proof mass, micro-mirror, micro-pump, pressure-sensitive membrane) needs to be free to move. Consequently, fabrication processes must be added to free the MEMS mechanical element. Additionally, since MEMS transducers are by design sensitive to some environmental influences, MEMS packaging must protect the transducer from undesired environmental influences. This results in packages that are more complex than those used in standard IC packaging.
There have been efforts to integrate the MEMS transducer with its sense electronics IC over the years. These have included packaging the MEMS and IC side by side, fabricating the MEMS directly on the IC, and stacking the MEMS and IC. The drawback to these approaches is that in general they require additional final packaging including a cap to protect the MEMS and wire bonds to make electrical connection to the IC. This chip-scale packaging adds considerable expense to the final device. The cap over the MEMS also makes chip stacking for 3DIC applications difficult, if not impossible.
An integrated motion processing unit has been proposed, however, the costs of individual chip packaging, board fabrication, chip mounting, and mechanical alignment make such approaches costly and useful mainly for high margin applications such as navigation.
Cost and size can be reduced by bringing more and more MEMS functions onto fewer MEMS chips and integrating more electrical functions into fewer IC's, and it is desirable to integrate all the MEMS sensors and electronics onto a single substrate. Typically, existing “single chip” inertial measurement units (IMUs) consist of bare chips that are adhesively attached and wire bonded together to a package substrate that is covered with a cap or plastic molding. Thus, beyond the system components within the chip, no further 3D integration is possible.